a) Field of the Invention
The present invention relates to an image pickup device, and more particularly to an image pickup device of a charge coupled device (CCD) type having a number of pixels disposed in a matrix pattern wherein a video signal is transferred in a CCD.
b) Description of the Related Art
Solid state image pickup devices of a CCD transfer type are known which are used in electronic camera, copiers, and other video apparatuses. A number of photodiodes are disposed in a pixel matrix having vertical columns and horizontal rows. Vertical charge transfer paths (VCCDs) are formed adjacent respective photodiode columns, and a horizontal charge transfer path (HCCD) is formed adjacent the ends of respective VCCDs.
For an electronic still cameras or the like using such a solid state image pickup device, there is a demand of exposing all photodiodes (PDs) at the same time and reading video signals separately.
In order to read electric charges stored in photodiodes and transfer them concurrently, it is necessary to use transfer pulses of three or more phases per one pixel or one row. In order to prepare transfer pulses of three or more phases per one pixel, it is necessary to form three or more electrodes per one pixel in a CCD. This is a disadvantage from the viewpoint of fine integration of the device.
A two-phase drive may be performed by forming a potential well and a potential barrier under each electrode. This approach poses a problem of inability of forming potential wells and barriers in a self-aligned manner at a manufacturing process.
As a transfer method eliminating the above disadvantages, an accordion transfer method has been proposed for an image pickup device having built-in photodiodes in a CCD (refer to Philips Technical Review, Vol. 43, No. 1/2, 1986 by A. J. P. Theuwissen and C. H. L. Weijtens).
With the accordion transfer method, it is possible to read all pixels by two electrodes per one pixel or row and to allow all pixels to be exposed at the same time by using an electronic shutter capable of substrate draining. Two electrodes per one pixel is the same number as used in an interline transfer (IT) CCD, a frame transfer (FT) CCD, and a frame interline transfer (FIT) CCD.
FIGS. 12A and 12B are diagrams explaining the accordion transfer method. FIG. 12A is a potential diagram explaining how the potential under successive electrodes of a transfer path changes with time. FIG. 12B is a schematic diagram in the form of a plan view explaining the motion of electric charges by the accordion transfer method.
Referring to FIG. 12A, electrodes on a transfer path include odd-numbered electrodes Od and even-numbered electrodes Ev. Under each electrode, a potential well or potential barrier is formed on a charge transfer path. The potential energy of an electron in the charge transfer path is diagrammatically shown by a solid polygonal line. The height of the polygonal line indicates the potential energy of electrons.
The electron's potential energy under each odd-numbered electrode is first lowered to form potential wells and store electric charges qa, qb, and qc. If the potential barrier between adjacent potential wells is lowered under this condition, electric charges in the adjacent potential wells are mixed together.
To avoid mixture of electric charges, the electron's potential energy under the rightmost even-numbered electrode is lowered to expand the potential well to the amount corresponding to two electrodes. The electric charge qa therefore moves to the right and distributes over the area corresponding to two electrodes. Next, the electron's potential energy at the left side area of the potential well storing the electric charge qa is raised, and at the same time the electron's potential energy at the right side potential barrier is lowered. As a result, the electric charge qa moves to the right by the amount corresponding to one electrode and distributes over the area corresponding to two electrodes.
Under this condition, the potential barrier corresponding to two electrodes are formed between the electric charges qa and qb. The electric charge qa can be transferred sequentially to the right side by raising the electron's potential energy at the left side and lowering the electron's potential energy at the right side in the above-described manner.
At the next timing after the potential barrier corresponding to two electrodes is formed between the electric charges qa and qb, the electron's potential energy of the potential barrier at the right side of the electric charge qb is lowered so that the electric charge qb is expanded to and distributed over the area corresponding to two electrodes.
At this time, there is a potential barrier corresponding to at least one electrode, and generally two electrodes, between the electric charges qa and qb, preventing a mixture of electric charges. In this manner, electric charges stored in the areas corresponding to every second electrode can be transferred while expanding and distributing them to areas corresponding to two electrodes with a gap of two electrodes.
FIG. 12B is a schematic diagram showing the distribution of electric charges transferred in the above manner. In FIG. 12B, the abscissa represents time, and the ordinate represents electrodes on a transfer path. Under the condition shown at the leftmost side, the electric charges qa, qb, qc, and qd are stored in the areas corresponding to every second electrode in the upper half of the transfer path. These electric charges are sequentially transferred to the lower areas of the transfer path, starting from the electric charge at the lowest area, by sequentially forming potential wells and barriers corresponding to two electrodes, i.e., by expanding the electric charge distribution over a two-fold area.
During the charge transfer, the electric charge distributes over the area corresponding to two electrodes, and the potential barrier corresponding to two electrodes is formed between respective electric charges. In this manner, electric charges stored in the areas corresponding to every second electrode can be transferred without a mixture of electric charges. Under the condition at the rightmost side in FIG. 12B showing the completion of charge transfer, the electric charges qa, qb, qc, and qd, are again distributed at every second electrode.
The generation of potential wells and barriers during the charge transfer is analogous to the operation of a musical instrument accordion when its bellows are gradually expanded and again compressed. This charge transfer method is therefore called an accordion transfer method. With this method, one signal per one photodiode row can be transferred using two electrodes per one row.
The present applicant has presented a domino type transfer method for a solid state image pickup device having a photodiode matrix, VCCDS, and a HCCD. This domino type transfer method performs the charge transfer like the accordion transfer method. However, instead of the frame transfer, electric charges read in the VCCDs are transferred to the HCCD while expanding them by a two-fold scale.
Briefly speaking, this method corresponds to the drive method at the upper half of the transfer path shown in FIG. 12B. Four-phase drive signals like the interline CCD are used. Also with this method, one signal per one photodiode row can be transferred.
It takes time to expand the charge distribution in order to transfer electric charges while expanding space between them. The time that a video signal stops on the transfer path differs greatly depending upon the position in the transfer path taken by the video signal. The transfer time required for each video signal is also different. A dark current is generated on the transfer path so that a large dark current is added to the video information staying on the transfer path during a longer time duration.
In order to compensate for such a dark current component, a light shielding area called an optical black (OB) area is provided to a CCD image pickup device. If a dark current of the same amount is generated in both the image pickup area and the OB area from which a dark current component is derived, the dark current component can be compensated.
FIGS. 13A to 13D are schematic diagrams explaining the function of an OB area. FIG. 13A is a diagram explaining an ideal function of the OB area. In this case, a dark current of the same amount is generated in both the image pickup area and the OB area. Although the base line of the black level is raised by the dark current, it is raised by the Same amount in both the image pickup area and the OB area. Therefore, by subtracting a black level at the OB area from a video signal at the image pickup area, the shading component can be compensated to thereby obtain real image information.
FIG. 13B is a diagram showing the base line of image information obtained in the above manner during one field. A real video signal obtained by subtracting a signal from the OB area from the video signal detected at the image pickup area, has a constant black value during one vertical scan period V.
The pixel structure of the image pickup area differs slightly from that of the OB area in that photodiodes in the latter area are covered with an aluminum film. The electrostatic capacitance at the OB area is therefore larger than that at the image pickup area, producing a difference of a dark current between-the image pickup area and the OB area.
FIG. 13C is a graph showing the base line of a real video signal. The base line for the OB area is lower than that for the image pickup area. As a result, an OB step is generated between the base line for the image pickup area and that for the OB area.
If a signal from the OB area is subtracted from such a video signal, a video signal with a certain amount of the OB step being added thereto is obtained. The OB step becomes larger as the time that the video signal stays on the vertical charge transfer path becomes longer.
FIG. 13D is a graph showing a change in the base line of a video signal during one field. A video signal read at the start of the vertical scan period V has substantially no OB step because of a short time that the signal stays on the vertical charge transfer path. On the other hand, a video signal read at the end of the vertical scan period has a large OB step because of a long time that the signal stays on the vertical charge-transfer path. Since the OB step increases with a lapse of time, the base line of a video signal changes as indicated by a broken line.
As described above, an OB area provided to a CCD image pickup device inherently generates an OB step which changes the black level of a video signal.